We consider in this note the excitation of ion-acoustic waves by vertical gradients of density in the ionosphere. The conclusion is reached that the fine structure of the ionosphere is probably affected by the resulting instability, as comparison with observations seems to indicate. Recently, Liu and Yeh [1966] have examined the excitation of low-frequency imcompressible waves in an inhomogeneous ionosphere. They find that, below the F-region maximum, there may exist instabilities, the growth rate being given approximately by grho'/rhonu, where g is the gravity, rho is the plasma density, rho' is its height gradients, and nu is the ion-neutral collision frequency. We have examined the stability of the ionosphere against growth of low-frequency quasielectrostatic waves, taking into account the compressibility of the plasma.